Method of heat treating steel



United States Patent METHOD OF HEAT TREATING STEEL Samuel Epstein andJoseph D. Dennison, Jr., Bethlehem,

Pa., assignors to Bethlehem Steel Company, a corporation of PennsylvaniaNo Drawing. Application July 14, 1954 Serial No. 443,428

Claims. (Cl. 14812.4)

This invention relates broadly to a novel treatment of open-hearth steelplates and the improved product resulting from such treatment.

More particularly, the invention relates to quenching semi-killed openhearth ship plates of A" thickness or greater from rolling temperaturesto obtain a microstructure comprising pseudo-martensite intermingledwith ferrite to lower the transition temperature in notched-bar impacttests of said plates.

The object of this invention is to obviate brittle fracturing in weldedships. Characteristics of such brittle fracturing are that the fracturestarts at imperfections in the weld, generally aggravated also by othernotch conditions, and that the fracture occurs in cold weather. Whilethe design of the ship and the workmanship are of the greatestimportance in obviating such brittle fracturing, recent work hasindicated that the quality of the steel also plays an important part.

It has recently been shown that ship plate steel which becomes brittlein notched-bar impact tests at relatively warm temperatures is morelikely to be the origin of brittle fracturing in a welded ship than shipplate steel which only becomes brittle in notched-bar impact tests atrelatively cold temperatures. The temperature at which such brittlenessensues in a series of notched-bar impact tests "ice example, .25% carbonand .45% to manganese.

We have discovered, however, that severe accelerated cooling, by waterquenching from the rolling temperature, of semi-killed open hearth shipplate steel, containing .05 to .20% carbon and ordinary manganesecontent, i. e., .25 to .60% manganese can give as low transitiontemperatures as are obtained upon ordinary rolling and air cooling ofthe recently developed semi-killed and fully aluminum-killed lowercarbon higher manganese ship plate steels, producing transitiontemperatures ranging from 0 to 70 F. in the unwelded condition and from20 to 50 F. in the welded condition.

The explanation of this appears to be that the severe acceleratedcooling by water quenching of the coarse grained steel present after theordinary finishing temperature in commercial rolling produces apseudo-martensitic structure With which ferrite is intermingled. Thisstructure of pseudo-martensite and ferrite produced by water quenchinghas a quite low transition temperature in notched-bar impact tests,considerably lower than the structure of ferrite and pearlite producedby ordinary cooling from the rolling temperature.

Such water quenched open-hearth semi-killed ship plate steel hasappreciably higher yield and tensile strength and lower transitiontemperature than such steel air cooled from the rolling temperature. Thelower transition temperature is evident not only in notched-bar impacttests of the unwelded steel but also in notched tests of the asweldedplate.

This is shown in the following data in Table I on semikilled open hearthship plate steel thick of the following composition:

O Mn P S Si Table 1 Tensile Properties Transition Temperature NotWeldedKeyhole Charpy Impact Test, F.

Welded N etched Weld-Bead Tensile Test, F.

Yield Strength,

p. s. i.

Tensile Strength, p. s. 1.

E1. in 2", Percent Red. of I Area, Percent As Air Cooled from theFinishing Temperature in Rolling, of 1,650 F As Water-Quenched for 15Seconds from the Same Finishing Temperature at continually loweredtesting temperatures hasbeen called transition temperature.

It has also recently been shown that ship plate steel of the semi-killedtype containing, for example, .18% carbon and .70% manganese, and thushaving a lower carbon content and a somewhat higher manganese contentthan ordinary ship plate steel, has a lower transition'temperature.Fully aluminum killed steel of the same lower carbon and highermanganese contents has an even lower transition temperature. Sincethicker ship plate is more susceptible to brittle fracturing thanthinner plate it has become the practice to use ordinary ship platesteel in thicknesses up to /2", semi-killed lower carbon highermanganese ship plate steel in thicknesses from /2" to 1", and fullyaluminum killed lower carbon higher manganese ship plate steel inthicknesses over 1. By ordinary ship plate steel is meant a steel havinga tensile strength of 58,000 to 71,000 lbs. p. s. i. Such steel maycontain, for

The data in Table I indicate that by proper choice of the carbon andmanganese content of the steel it is possible to stay Within the yield,tensile, and elongation requirements in the tensile test for ordinaryship plate steel (yield strength 32,000 p. s. i. minimum, tensilestrength 58,000 to 71,000 p. s. i., elongation in 2" 22% minimum) afterwater quenching.

The increased yield strength and tensile strength after Water quenchingare of marked advantage. The decreased transition temperature producedby the water quenching indicates the water quenching to be of definitehelp in obviating brittle fracturing. At present two new special classesof ship plate steel are required by the American Bureau of Shipping,Class .B, maximum carbon content 0.23%, manganese content 0.60 to 0.90%,semi-killed steel for plates /2 to 1" thick, and Class C ofapproximately the same composition but of fully aluminum-killed steelfor plates over 1" thick. Both of these classes of steel require higherthan ordinary manganese content. While manganese is generally not verycostly it may yet become scarce during a period of great activity insteelmaking and/or shipbuilding. Class C steel furthermore being fullyaluminum-killed steel made in hot-topped big- 5 end-up molds may alsorequire additional hot-topping and stripping facilities during such aperiod but our waterquenched plate of low carbon semi-killed steel inwhich the manganese content has not been increased shows transitiontemperatures in notched-bar tests as low as the new special classes of Band C steel with their increased manganese contents. This is true of theunwelded steel and also of the as-welded steel.

This is indicated by the following data in Table II for Class B andClass C steel, which may be compared with the data in Table I for oursemi-killed open hearth steel is still quite warm, amply so to aid insubsequent straightening. However, it longer quenching times than thisare used no great harm is done as the transition temperature is just aslow after complete quenching to the water temperature. The steel canthen be tempered if desired to as high as 1000 F. without raising thetransition temperature; such tempering lowers the yield and tensilestrength somewhat by approximately 5000 p. s. i.

The lowered transition temperature after water quenching is obtainedover a wide range of finishing temperatures in rolling. As lowtransition temperatures are obtained after water quenching from a highfinishing temperature in rolling of l850 to 1900 F. as from a rather lowfinishing temperature in rolling of 1600 to 1650 F. Such quenchingshould, however, be from above the critical temperature of the steelbeing rolled.

Table II Tensile Properties Transition Temperature Not Welded WeldedYield Tensile El. in 2" Red.of Strength, Strength, percent Area,

p. s. i. p. s. 1. Percent Keyhole Notched Charpy Weld-Bead ImpactTensile Test, F. Test, F.

Class B Steel, Air Cooled from the Finishing Temperature in Rolling (C0.19%, Mn 0.71%, Si 0.05%) Semi-Killed 35, 000 62, 000 32. 0 62. 0 7 23Class 0 Steel, Also Air Cooled as Above (C 0.17%, Mn0.73%, Si0.21%)Fully Aluminum-Killed 37, 000 62, 000 32.0 61.0 33 12 We claim:

which has been water-quenched from the rolling temperature.

The water-quenched semi-killed steel of Table I shows better yieldstrength, tensile strength, and transition tem- 4o perature in theunwelded condition and after welding than the new special Class C lowercarbon higher manganese fully aluminum-killed steel.

Regarding the preceding data, the transition temperature in thenotched-bar impact test was obtained in the usual way with standardCharpy specimens with keyhole notches, and with the transitiontemperature read when the impact resistance vs. temperature curvecrossed ft. lbs. as the testing temperature was lowered.

The specimens for the notched weld-bead tensile tests were made asfollows. A longitudinal specimen, the full thickness of the /1" plate,12" long and 2.6" wide, was welded by laying a narrow weld beadlongitudinally along the center of one of the flat surfaces, using a 7in. diameter E-60l0 electrode, 175 amperes, 28 volts, and 12 in. travelper minute. The weld bead was then notched by machining the notchperpendicular to the head down to the plate surface. The notch was madequite sharp, the radius at the base of the notch being 0.0015 in. Thespecimens were then loaded in tension to fracture at a series of loweredtemperatures and a transition temperature obtained by measuring thereduction in thickness beneath the notch of the fractured specimens. Thetransition temperature was read when the reduction in thickness vs.temperature curve crossed 5% reduction of thickness as the testingtemperature was lowered.

In water-quenching the plate steel from the rolling temperature thesteel is led, for example, into a large tank of cold water, the waterbeing preferably of a temperature of from to 120 F., after passingthrough the finishing rolls. The plate being quenched is held in the 70water tank for the time necessary to produce the structure ofpseudo-martensite intermingled with ferrite, preferably from 5 to 60seconds. As has been shown, 15 seconds immersion of /1" thick plate inthe water at 60 F. is sufiicient for this. After such quenching theplate 75 1. A method of lowering the transition temperature in thenotched bar impact test of semi-killed open-hearth steel platecontaining .05% to .20% carbon, .25% to .60% manganese, and the balanceconsisting essentially of iron, comprising hot rolling said plate andwater quenching the said steel plate immediately from the hot rollingoperation, the temperature at which said plate is quenched being abovethe upper critical temperature of the said steel plate.

2. A method of lowering the transition temperature in the notched barimpact test of semi-killed open-hearth steel plate containing 05% to.20% carbon, .25% to .60% manganese, and the balance consistingessentially of iron, comprising hot rolling said plate and directlywater quenching said plate from the hot rolling operation attemperatures of from 1600" to 1900 F.

3. A method of lowering the transition temperature in the notched barimpact test of semi-killed open-hearth steel plate containing .05% to.20% carbon, .25% to .60% manganese, and the balance consistingessentially of iron, comprising hot rolling said plate and directlyquenching said steel plate from the rolling operation at temperatures offrom 1600 to 1900 F. in water, said water being at temperatures of from40 to F.

4. A method of lowering the transition temperature in the notched barimpact test of semi-killed open-hearth steel plate containing .05% to.20% carbon, .25% to .60% manganese, and the balance consistingessentially of iron, comprising hot rolling said plate and directlyquenching said steel plate from the rolling operation at temperatures offrom 1600 to 1900 F. for from 5 to 60 seconds in water, said water beingat temperatures of from 40 to 120 F.

5. A method of lowering the transition temperature in the notched barimpact test of semi-killed open-hearth steel plate containing .05% to.20% carbon, .25% to .60% manganese, and the balance consistingessentially of iron, comprising hot rolling said plate and directlywater 5 quenching the said steel plate from the hot rolling operation.

References Cited in the file of this patent UNITED STATES PATENTS PierceAug. 7, 1906 Kenney Nov. 28, 1916 6 OTHER REFERENCES Reed:Photomicrographs of Iron and Steel, 1929, page 63

1. A METHOD OF LOWERING THE TRANSITION TEMPERATURE IN THE NOTCHED BAR IMPACT TEST OF SEMI-KILLED OPEN-HEARTH STEEL PLATE CONTAINING .05% TO .20% CARBON, .25% TO .60% MANGANESE, AND THE BALANCE CONSISTING ESSENTIALLY OF IRON, COMPRISING HOT ROLLING SIAD PLATE AND WATER QUENCHING THE SAID STEEL PLATE IMMEDIATELY FROM THE HOT ROLLING OPERATION, THE TEMPERATURE AT WHICH SAID PLATE IS QUENCHED BEING ABOVE THE UPPER CRITICAL TEMPERATURE OF THE SAID STEEL PLATE. 